This invention relates to actuator materials, and more particularly relates to materials that can demonstrate an actuation response to an applied external stimulus such as an applied field stimulus.
The ability to effectively employ actuator materials for producing motion and force in response to an applied stimulus is becoming increasingly important for advanced transportation and aeronautics applications, advanced automation and manufacturing processes, and a wide range of other fields. Of particular interest is the development of actuation materials having large strains, appreciable force generation, and rapid time of response to an external stimulus. Popular classes of actuation materials include piezoelectric, magnetostrictive, and shape memory actuation materials; each of these three classes has been found to exhibit both performance advantages as well as limitations in actuation capabilities.
Piezoelectric materials are typically ceramic materials, e.g., lead-zirconate-titanate, and are characterized by an ability to mechanically deform, i.e., expand and contract, in response to an applied electric field, in a demonstration of the inverse piezoelectric effect. Piezoelectric ceramic actuation members, conventionally employed in series in a stack form, exhibit an acceptable output energy density as well as a very high bandwidth, i.e., a relatively fast actuation stroke. A piezoelectric stack structure is generally limited, however, to only a relatively small stroke, and can typically produce only a limited output force, largely due to the characteristic brittleness of piezoelectric materials. As a result, stroke and force amplification mechanisms are often required of an actuator incorporating a piezoelectric actuation material, but for many applications, the limited piezoelectric actuation force cannot be rendered sufficient for the application as a practical matter.
Magnetostrictive actuation materials typically are characterized as being capable of producing an actuation force and an actuation stroke that are greater than that capable of piezoelectric materials. Application of a magnetic field to a magnetostrictive material causes the material to be strained as the domain magnetization vectors of the material rotate to align with the direction of the applied magnetic field. The unit cells of the material are strained by the magnetization rotation but their orientation is not changed.
Rare-earth alloys are found to exhibit the largest magnetostrictive strains; e.g., the Laves phase, TbFe.sub.2 exhibits a magnetostriction, .lambda..sub.s, of about 1753.times.10.sup.-6 at room temperature in an applied field of about 25 kOe, and the near-zero pseudobinary, Tb.sub.0.27 Dy.sub.0.73 Fe.sub.2, known commercially as Terfenol-D.RTM., exhibits an easy axis magnetostriction, .lambda..sub.111, of about 1600.times.10.sup.-6 in fields on the order of about 1 kOe. The term "easy axis" is used herein to refer to the crystallographic axis along which a magnetization vector of a domain prefers to lie in the absence of an applied field; the easy axis thus is the direction of easy magnetization for a domain.
The strains, and corresponding forces, developed by Fe--Dy--Tb intermetallics such as Terfenol-D.RTM. can be considerably higher than those of piezoelectric materials, and the energy densities of such intermetallics can be an order of magnitude greater than conventional actuation systems such as hydraulic systems. The strain in unstressed Terfenol-D.RTM. exhibits a field sensitivity of about 0.6.times.10.sup.-6 per Oe under a magnetic field of about 1,000 Oe and even larger strain sensitivity to an applied field can be enabled in pre-stressed material configurations.
While magnetostrictive actuation elements do exhibit a relatively high-frequency actuation response, they are fundamentally limited by their electrical conductivity, which precludes operation at very high actuation frequencies due to the formation of eddy current in the material in response to a changing applied magnetic field, unless at least one of the material dimensions of the elements perpendicular to the field is small. An additional limiting constraint of magnetostrictive materials is that they typically are characterized by an actuation stroke that, like that of piezoelectric actuation elements, is limited in its extent, here due to the domain elongation inherent in the actuation mechanism.
The class of actuator materials known as shape memory alloys is characterized in that when plastically deformed at one temperature or stress condition in a phase known as the martensitic phase, a shape memory alloy can recover its original shape when subjected to an alloy-specific martensitic-austenitic transformation temperature or stress condition that reverts the material to a corresponding parent, austenitic phase. This effect is based on the restoration of twin variants of the martensite phase of the material to their austenitic shape. Such materials are capable of reversing a large stress-induced martensitic deformation when transformed back to the austenitic phase, and thus can enable a large actuation stroke mechanism. Furthermore, the recoverable strain accommodated by a shape memory alloy is generally considered to be the largest achievable for any actuation material, and can be as large as about 20%, for, e.g., the Cu--Al--Ni alloy.
The large stroke generally characteristic of shape memory alloys is offset by the typically very slow actuation response time of the materials when the martensitic/austenitic transformation is thermally controlled. As a result, shape memory actuation can not accommodate applications requiring even moderately high actuation frequencies. Furthermore, the shape memory transformation is generally characterized as a poor energy conversion mechanism; much of the heat supplied to the material to drive the martensitic/austenitic transformation is uncontrollably lost to the surroundings. Thermal control of the shape memory effect also limits the allowable operational temperature range of an application for which a shape memory alloy is to be employed.
For many actuation applications, it is ideally preferred to combine the large actuation stroke provided by shape memory alloys with the fast actuation response time of magnetostrictive and piezoelectric materials. At the same time, the thermal constraints of shape memory, piezoelectric, and magnetostrictive materials are also preferably eliminated. A single actuation material embodying all of these qualities has heretofore not been realizable under practical operating conditions.